Study On The Mechanism Of Particle Breakage Of Coral Debris

In recent years,China has accelerated the construction of coral reef foundation projects,among which coral reef sand and reef limestone provide valuable building materials for construction.Coral debris can be used to build island on the stable cay.When building the reef foundation construction,a correct understanding of the engineering mechanical properties of coral debris materials(the relationship between particle strength,particle breakage and particle type characteristics)will provide theoretical and technical support for the engineering practice of hydraulic land reclamation sites,airport runway foundations and other geotechnical structures,as well as plain concrete structural components.In this paper,the automatic particle strength tester and rock strength point load tester were used to study the particle strength of two types of coral debris in under different conditions in the construction of islands and reefs.At the same time,the discrete element numerical simulation software(PFC3D)was used to simulate the relationship between particle breakage and particle shape characteristics of triaxial samples under pressure.The results show that:(1)The coral debris material has a small specific gravity.The proportion of newly broken mineral particles in reef limestone(hereinafter referred to as reef rock particles)is generally smaller than that of biological debris particles(hereinafter referred to as debris particles).Moreover,the average water absorption rate of the reef rock particles is as high as 21.90%,nearly twice as high as that of debris particles.The results show that the connectivity between the interior pores and the surface of the particles is better than that of the debris particles,which provides a precondition for the softening of the particles with wet water.(2)Coral clastic particles have the characteristics of low hardness and high moisture absorption.Reef rock strength is not only significantly lower than debris particles.The strength softening characteristics(softening and decreasing by 1/3)of the reef-rock particles after wetting and absorbing water are relatively higher than those of debris particles.Coral clastic particles show a negative correlation with particle size,which conforms to Weibull statistical characteristics of brittle fracture strength distribution.The particle strength of coral debris is obviously lower than that of conventional sand.But the Weibull modulus m of the distribution of coral debris particle failure strength is low(generally <5).Obviously,the abundant microporous tubes and micro-cracks in the coral particles are the main internal mechanism of the brittle failure of the particles.(3)Discrete element numerical simulation of the uniform sample analysis shows that simulation of triaxial shear test to get the internal friction Angle of 25 °~40°.The results show that the cohesive force is caused by particle breakage in the shear process.In particular,the cohesive force generated by the simulated compacted sample reached 164.49 kPa.According to Moore strength envelope of a given sample of shear strength parameters c and ?,compacted specimen failure surface and the major principal stress Angle is 65 °.The characteristics of high cohesion strength are not only consistent with the characteristics of high cohesion strength of indoor unit samples,but also verify the working mechanism of broken and dilatancy strength components of coral sand particles at low stress level.The crushing finally concentrates on the location where obvious swelling occurs.The development of particle breakage and the relationship between particle characteristics and particle breakage conform to the results of laboratory tests and the theory of physics and mechanics.(4)The discrete element simulation of irregular granule samples has limitations in the construction of granule type and the calculation capacity of the system.The irregular granule of coral sand is easier to break up.As a result,the characteristics of concentrated evolution in the direction of sample crushing are weakened,the displacement distribution of sample particles is more uniform and the range of rotation is larger.The strength characteristics of the sample system are relatively reduced.However,at a low level,the comprehensive strength component formed by the crushing and recombination of coral sand and dishing remains relatively high(about c=90kPa).(5)The microscopic characteristics of discrete element numerical simulation show that the number of triaxial shear particle breakage is positively correlated with stress level,shear strain and initial state.In the case of vertical stress increment,the mechanical mechanism of particle compaction is superior to that of particle compaction.The interparticle of the dense sample bears the shear stress directly,and the particle breakage occurs earlier and in larger quantity,and is concentrated in the dominant area of the shear zone.The macroscopic instability characteristics of the sample are formed.The crushing tendency of sample particles is first concentrated on the relatively weak crushing dominant surface.When the development of particle crushing in this direction is relatively saturated,a large number of other crushing directions begin to appear.The dominant zone of shearing zone is broken,and the grain rotation is dominant and the translation is small.Local void increases and coordination number decreases.